The present invention relates to a receiver for satellite broadcasting, more particularly to a polarotator pulse generator circuit which controls a polarotator for changing an angle of a receiving antenna probe for satellite broadcasting. In order to receive a multichannel electric wave transmitted from a satellite in a satellite broadcasting receiver, the angle of the probe attached to an antenna of the receiver according to a deflection of the electric wave needs to be changed by using a polarotator.
Generally, for driving the polarotator in a satellite broadcasting receiver, there is needed a pulse as shown in FIG. 5 (a), which has a period T of 16-21 msec and a width W of minimum 0.7 msec and maximum 2.3 msec.
When the pulse as shown in FIG. 5 (a) is applied, the polarotator controls an angle of the probe attached to an antenna of a satellite broadcasting receiver from 0 to 180 degrees according to a change of the pulse width. For example, supposing that a minimum pulse width W 0.7 msec corresponds to 0 degree, a maximum pulse width W 2.3 msec corresponds to 180 degrees for which the pulse period T must be in the range of 16 to 21 milliseconds.
That is, in order to control a probe of the antenna according to the deflection of the received electric wave in the satellite broadcasting receiver, a PWM (pulse width modulation) signal is needed and the power supply and ground level must be also transmitted to the polarotator.
The conventional polarotator pulse generator circuit is composed as shown in FIG. 1 to generate the PWM signal.
In FIG. 1, a control voltage input part 1 for converting an applied square wave signal to a linear control voltage comprises resistors R3, R4, R5 and capacitors C2 and C3 which charge and discharge an applied voltage according to a RC time-constant.
A charging/discharging part 2 controls a timer IC by receiving a DC voltage, in which the DC voltage is connected to a charging resistor R1, and a discharging resistor R2 and a diode D1 are connected to the charging resistor R1 and to the capacitor C1. At this time, the resistor R1 is valued much lower than that of R2.
Said timer IC connected to the control voltage input part 1 and the charging-and-discharging part 2 to provide the control square output to the polarotator is composed as shown in FIG. 2.
In a comparing part 3, an output of the diode is applied to a noninverting terminal+ of a comparator CP1, while the output of the control voltage input part 1 is applied to an inverting terminal- of a comparator CP1. Also, a power supply voltage VCC is connected to the inverting terminal- of the comparator CP1 through a voltage dividing resistor RA1 among resistors RA1, RA2 and RA3 having the same resistance value.
When there is no voltage applied from the control voltage input part 1 to the inverting terminal- of the comparator CP1, if the output of the charging and discharging part 2 is more than 2/3 VCC, the comparator CP1 provides a high level output.
On the other hand, the discharge voltage of the capacitor C1 in the charging and discharging part 2 is applied to an inverting terminal- of a comparator CP2, while the power supply voltage VCC is applied to a noninverting terminal+ of a comparator CP2 through the voltage dividing resistors RA1 and RA2, so that when there is no voltage applied from the control voltage input part 1, if the discharge voltage of the capacitor C1 is below 1/3 VCC, the comparator CP2 provides a high level output.
An output part 4 for providing a square wave control signal to a polarotator according to the outputs of the comparing part 3, comprises a RS flip-flop, a switching transistor Q1, and an inverter I1. The outputs of the comparators CP1 and CP2 are applied to the reset and set terminals R and S of the RS flip-flop respectively. The output of the RS flip-flop is applied to the switching transistor Q1, thereby controlling the charge and discharge of the capacitor C1 according to the driving of the transistor Q1. Also, the output of the RS flip-flop is inverted by the invertor I1 and the inverted output is provided to control the polarotator.
In this conventional polarotator generator circuit, if the output of the pin 3 of the timer IC is high, since the output Q of the RS flip-flop provides a high level signal, the transistor Q1 is turned off. In this case, a DC voltage is charged to the capacitor C1 through the resistor R1 and the diode D1, for which if the charged voltage of the capacitor C1 is larger than 2/3 VCC, the comparator CP1 becomes high level signal and is applied to the reset terminal R of the RS flip-flop. At this time the RS flip-flop is reset and the output terminal Q of the RS flip-flop provides a low level signal, so that the transistor Q1 is turned on and pin 3 provides a low level signal.
With the transistor Q1 turn-on, the voltage charged in capacitor C1 is discharged through the resistor R2 and if the voltage of capacitor C1 becomes below 1/3 VCC, the comparator CP2 applies a high level signal to the set terminal S of the RS flip-flop. Thus, the RS flip-flop is set and the output Q of the RS flip-flop provides a high level signal, thereby turning off the transistor Q1. With the transistor Q1 turn-off, the pin 3 provides a high level signal, so that the above operation is repeated from the start.
That is, the pulse width W, a logic high state in FIG. 5 (a), is determined by a time spent for the voltage across the capacitor C1 to increase from 1/3 VCC to 2/3 VCC through the diode D1 and the charging resistor R1. The difference of the pulse period and the pulse width, T-W in FIG. 5 (a), is determined by a time spent for the voltage across the capacitor C1 to fall from 2/3 VCC down to 1/3 VCC through the discharging resistor R2.
Since the polarotator pulse has the smaller pulse width W than the pulse period T, when the capacitor C1 charges, the diode D1 is used in parallel with the resistor R2 to avoid the resistor R2. At this time, the resistance value of R1 is much smaller than that of R2 so that the charge time can be reduced, Thus, there is always provided the pulse from to the pin 3 of the Timer IC.
If a control voltage is used to change the reaching voltage of the capacitor more or less than 2/3 VCC, the time of pulse width W for the capacitor C1 to increase from 1/3 VCC to the controlled reaching voltage is adjusted, thereby obtaining a desired pulse width.
However, the control voltage must be DC voltage, so that the MICOM (microcomputer) must employ a digital-to-analog converter for changing the level of DC voltage in MICOM, making the circuit more complex.
Thus, without the use of the digital-to-analog converter, the pulse width is changed by using a pulse width modulating port of the MICOM and the pulse width modulated output signal is converted to the DC control voltage by using the capacitors C2 and C3. But, although the pulse width modulated output signal changes linearly, the extracted DC voltage does not change linearly.
Also, the pulse width W of the polarotator signal does not change linearly, so that the polarotator rotates fast the probe of the antenna at first, but the polarotator rotates slowly toward the end of rotation. It results that the rotation speed of the probe is not constant.